1. Field of the Invention
The present invention relates to a method for manufacturing a spark plug and an apparatus for carrying out the same.
2. Description of the Related Art
Conventionally, in manufacture of a parallel-electrode-type spark plug, a spark gap is formed and adjusted in the following manner: after a ground electrode is subjected to preliminary pressing, the ground electrode is repeatedly subjected to pressing while the gap is monitored by use of a CCD camera or a like device, until the gap reaches a target value.
3. Problems to be Solved by the Invention
In using a method for calculating a gap on the basis of image information obtained through photographing a tip portion of a spark plug by use of photographing means, such as a CCD camera, the edge of a center electrode and that of a ground electrode must be photographed accurately and sharply in order to obtain the value of a gap at high accuracy. An effective method for attaining this end is as follows: a tip portion of a spark plug is irradiated with light emitted from illumination means which is located opposite the photographing means with respect to the tip portion, to thereby produce a sharp silhouette of the electrodes.
However, when, as mentioned above, light is emitted from the illumination means which is located opposite the photographing means with respect to the tip portion of the spark plug, illumination rays which are directed toward the photographing means in such a manner as to diverge from a spark gap irradiate edge portions of the electrodes due to diffraction after passing through the spark gap. As a result, the edge portions appearing in an obtained image may lose sharpness.
An object of the present invention is to provide a method for manufacturing a spark plug, capable of photographing a center electrode and a ground electrode such that edges of the electrodes are detected at high accuracy, and capable of manufacturing a spark plug at high accuracy on the basis of a photographed image of the electrodes, as well as to provide an apparatus for carrying out the same.
The above object of the present invention is achieved by providing a method for manufacturing a spark plug comprising a center electrode disposed within an insulator, a metallic shell disposed outside the insulator, and a ground electrode, one end of the ground electrode being joined to an end face of the metallic shell, an opposite end portion of the ground electrode being bent such that a side surface of the opposite end portion faces an end face of the center electrode so as to form a spark gap between the side surface and the end face, the method comprising:
a photographing step for photographing the spark gap by use of photographing means disposed at a position for receiving illumination rays which are emitted from illumination means so as to be restrained in divergence from the spark gap and which have passed through the spark gap; and
an after-treatment step for performing a predetermined treatment on the basis of image information obtained from the photographing step.
The present invention further provides an apparatus for carrying out the method.
In more detail, a light shield can be used to block out illumination rays which are directed toward the photographing means so as to diverge from a spark gap after passing through the spark gap, thereby effectively preventing reflections of the illumination rays (i.e., effectively preventing reflections from an edge portion of the surface of the center electrode facing the photographing means and reflections from an edge portion of the surface of the ground electrode facing the photographing means). Specifically, as shown in FIG. 12(a), illumination rays which enter the spark gap at a greater incident angle (specifically, at a greater incident angle with respect to a direction perpendicular to the axial direction of the center electrode) are more likely to directly irradiate an exit edge portion of an electrode when passing through the spark gap. Thus, the diffraction of illumination rays passing through the spark gap becomes noticeable toward the surfaces of the electrodes facing the photographing means. As a result, the image thus obtained loses sharpness of the edge portions. In FIGS. 12(a) and 12(b), the x direction is the direction along which the illumination means and the photographing means face each other, and the y direction is the axial direction of the center electrode.
The light shield can also be a mask having an aperture having either a fixed or variable opening. A collimating lens can be used in combination with a mask having an aperture in a preferred embodiment, where the mask blocks diverging illumination rays.
The above-described method enables easy adjustment of the axial distance of an emission region through which illumination rays are emitted. The axial distance of the emission region can be adjusted so as to be suited for the spark gap. Since illumination rays which enter the spark gap at a large angle can be blocked out, only parallel rays as shown in FIG. 12(b) or near parallel rays pass through the spark gap, thereby restraining reflections of diffracted rays from the edge portions of the electrodes facing the photographing means. The image thus obtained provides highly accurate silhouettes of the center and ground electrodes, thereby enabling an accurate value of the gap to be determined.
Alternatively, illumination rays may be emitted so as to pass by a light shield disposed between the spark gap and a light source provided on the illumination means. Emission of illumination rays via the light shield enables a desired illumination range to be attained regardless of the size of the light source. Further, the following arrangement may be employed: light shields are disposed along the axial direction of a spark plug on axially opposite sides of the spark gap so as to define therebetween an emission region for allowing illumination rays to pass through; and the distance as measured along the axial direction between the edges of the light shields which face the emission region is adjusted to 0.5 mm to 30 mm. This adjustment effectively restrains reflections of diffracted rays from the edge portions of the electrodes while maintaining a sufficient quantity of light for obtaining an image of the center and ground electrodes. When the axial distance of the emission region is less than 0.5 mm, the quantity of light is insufficient for obtaining the image. When the axial distance is in excess of 30 mm, illumination rays which enter the spark gap at a large incident angle increase, potentially causing an increase in the amount and range of reflections from the exit edge portions of the electrodes. Such adjustment prevents these problems.
Further, parallel rays emitted from a parallel-ray emission means may be employed so as to restrain divergence of illumination rays from the spark gap. Since illumination rays enter the spark gap at an incident angle of substantially zero, illumination rays that directly irradiate an exit edge portion of an electrode can be greatly reduced, thereby effectively restraining diffraction of rays around the edge portion.